the mystery of Vanessa unidirectional migration

Chris J. Durden drdn at
Tue May 1 01:05:47 EDT 2001

I think the whole story can be compared to the much better known locust 
migrations out of Africa. I think the reasons are the same. A good year in 
marginal habitat exhausts larval food supply. The population must move out. 
By moving out it can repopulate marginal habitats that periodically have 
gone extinct. The core habitat of the range may then be repopulated by 
wanderers from the marginal habitats. Although the mass of individuals move 
out in one direction (in this case NE across Colorado, then perhaps to the 
NE US) some individuals move out in all directions, even S. The species may 
often (not always) be found on isolated mountain grasslands across Mexico 
and into Central America.
    I have yet to see one here in Central Texas this year. Some years we 
have no *V. cardui* Painted Ladies at all! We do however have many *V. 
virginiensis* APLs.
..............Chris Durden

At 08:57 PM 4/30/2001 -0500, you wrote:
>I don't understand what advantage this bug gains by migating north every 
>spring, if the adults don't then migrate back south in the fall, and (?) 
>thus their offspring are instead killed off by winter.  Isn't this genetic 
>suicide?   I understand dispersion is good, but it seems like consistent 
>directional dispersion in a direction that is ultimately death would be 
>selected against, eventually.   What is going on?  I must not be seeing 
>the whole picture.
>Liz Day
>Indianapolis, Indiana, central USA  (40 N, ~86 W)
>USDA zone 5b.  Winters ~20F, summers ~85F.  Formerly temperate deciduous 
>daylight at
>"It is quite remarkable, when you think of it, that if you tell somebody to
>buy something and dump it on or squirt it on, he will almost certainly do
>it, after a fashion. But if you suggest that he observe something or think
>about something or learn about something, he almost certainly will
>not. Yet those gardens we admire are never the results of dumping and
>squirting: they are always the result of muddling things about in the brain
>and the eye."
>-- from _The Essential Earthman_, by the late Henry Mitchell
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